In a fixing device, a drying device, an erasing device, and a printing device of an image forming apparatus using an electrophotographic printing method, a fixing member and a pressure member which are rollers or belts sandwich and feed a printing medium on which an unfixed image or a printed image is formed with a developer (toner, etc.). While the printing medium is being fed, the unfixed image or the printed image is melted or dried by heat, so that the unfixed image or the printed image is, for example, fixed to the printing medium.
As shown in FIG. 2 which explains the present embodiment, for example, the image forming apparatus using the electrophotographic printing method includes (i) an image forming section which includes a photosensitive drum 1 and respective means provided around the photosensitive drum 1 and (ii) a fixing section which is provided downstream of the image forming section in a feeding direction of the printing medium and includes a fixing device 14. In the image forming section, a toner image is formed on the photosensitive drum 1, and then the toner image is transferred to the printing medium. In the fixing section, the printing medium is sandwiched by a nip portion between the fixing member (fixing roller 31) and the pressure member (pressure roller 32), and while the printing medium is being fed, the toner image is heated and melted so that the toner image is fixed on the printing medium.
Here, if (i) a reverse polarity toner (a reverse polarity developer) adheres to, for example, the back surface of the printing medium in the image forming section, and (ii) an electrostatic force (electrostatic force in a direction of the pressure roller 32) with respect to the toner of the fixing roller 31 or the pressure roller 32 is stronger than releasability with respect to the toner, the reverse polarity toner melts and adheres to the pressure roller 32. Moreover, the reverse polarity toner adheres to a second heating member, a cleaning member, etc. which are in contact with the pressure roller 32, and further the reverse polarity toner adheres again to the fixing roller 31 or both surfaces of the printing medium. Thus, an image failure occurs on an image of the printing medium.
Moreover, if the amount of toner (reverse polarity toner) which is not fixed onto the printing medium but is accumulated on respective means increases, it becomes impossible to secure functions of those means, and this interferes a normal image forming operation. Moreover, it becomes impossible to maintain an image-forming function over the long term, and the life of the means become extremely short.
Here, as a conventional technique, Japanese Patent Publication No. 2,734,146 (published on Aug. 19, 1991) discloses a technique of preventing contamination caused by the toner adhered to the fixing member. This conventional technique prevents the contamination by (i) applying to the fixing member a bias voltage having the same polarity as the toner and (ii) defining a volume resistivity of a surface release layer of the fixing member.
However, the technique disclosed in this Publication does not have a satisfactory function of preventing the contamination caused by the reverse polarity toner adhered to the back surface of the printing medium. Therefore, the problem of the contamination by the reverse polarity toner still exists, and it is still impossible to maintain the function of the image forming apparatus over the long term.
Therefore, an object of the present invention is to provide a fixing method, a fixing device, and an image forming apparatus which can prevent the image failure caused by the reverse polarity toner, maintain the normal image forming operation, and secure a satisfactory image quality and the life of each means over the long term.